In practice of magnetic recording, analog recording has been being replaced by digital recording which provides digitally-encoded information that is more durable. Digital magnetic recording media and systems are required to be not only superior in image and sound quality but compact and space-saving. Digital recording generally needs more signals than analog so that magnetic recording media for digital recording are required to enable higher density recording.
A magnetoresistive (MR) head for reading based on MR effects has recently been extending its use. An MR head provides a few times as much output as an inductive head. Besides, an MR head achieves great reduction of noise created by equipment, such as impedance noise. This means that reduction of noise of a magnetic recording medium per se enables satisfactory signal reproduction and brings about remarkable improvement on high density recording characteristics. Solutions for minimizing the noise of a magnetic recording medium per se include particle size reduction of ferromagnetic powder and smoothing the magnetic layer surface.
Higher in-plane recording density in magnetic tape can be achieved by shortening the wavelength of recording signals thereby to increase the recording density in the longitudinal direction and by decreasing the recording track width thereby to increase the recording density in the width direction. To realize this, particle size reduction of ferromagnetic powder, increase in packing density, and smoothing the magnetic layer surface are demanded.
Changes in environmental temperature or humidity cause a magnetic tape medium to extend or contract in its width direction. Where magnetic tape has a small track width, such expansion or contraction can cause off-track errors. This problem has generally been settled by reducing the thermal and hygroscopic expansion coefficients of the tape to such a level at which an off-track distance may not exceed several micrometers. This is because the difference between write track width and read track width in conventional write/read systems is more than 10 μm (about 105 to 13 μm). Since the write track width is sufficiently wider than the read track width, and the read track runs within the width of the write track, an off-track distance of several micrometers, if any, does not lead to output reduction.
However, as the write track width becomes smaller to cope with the demand for higher density recording, an off-track distance as small as about several micrometers will not be negligible. In other words, there has arisen a need to more strictly control tape expansion and contraction in the width direction against changes in environmental temperature and humidity conditions.
To meet the need, it is necessary not only to control the thermal and hygroscopic expansion coefficients of a nonmagnetic support of magnetic tape but to determine and control other physical properties of the nonmagnetic support that are influential on tape expansion and contraction. Related art concerning a magnetic recording medium of which the nonmagnetic support has any controlled physical property other than thermal and hygroscopic expansion coefficients includes a magnetic recording medium of which the nonmagnetic support is made of polyethylene naphthalate with a controlled molecular weight and a controlled density and a magnetic recording medium of which the nonmagnetic support has an optimized Young's modulus (see, for example, JP-A-7-6351 and JP-A-8-45060).
The techniques disclosed in JP-A-7-6351 and JP-A-8-45060 aim to secure the strength of a nonmagnetic support while preventing the raised edge (or high edge) phenomenon that occurs on slitting a tough film into tape, thereby to obtain a magnetic recording medium with high running durability. These publications are silent on reduction of an off-track distance in a write/read system having a reduced track pitch. Neither do they have a mention of the surface properties of the medium. Poor surface conditions of a magnetic tape result in a poor tape pack (poor tape pack wind quality), which deteriorates the running durability. The supports according to the publications cited above are inadequate for use in the latest high recording density magnetic recording media.